Magnet wire or electrically insulated lacquered wire is widely used in the fields of electronics and telecommunication as well as in electric motors and transformers. This wire is formed with generally a copper or aluminum conductor surrounded by a thin but very elastic and heat resistant coat of synthetic resin.
The lacquered wire is made by continuously repeated coatings of a lacquer on the conductor in a lacquering apparatus. The lacquer is basically an organic synthetic resin solution. Following each coating with the lacquer, the wire is heated to a temperature between 300 and 550.degree. C - the exact temperature being chosen after consideration of the lacquer base, the diameter of the wire and the rate of discharge of the wire from the lacquering apparatus. This heating hardens the film of lacquer while evaporating the solvent. The fumes of the solvent are removed by a ventilator.
In order to produce a smooth, bubble- and solvent-free film, it is best to maximize the number of coatings while keeping each coat thin.
There are various types of insulated wires; some of the more important types are defined by DIN (German Industrial Standards) 46416, sheets 1-5. On sheet 2, DIN 46416 specifies the standards for directly tinnable insulated copper wires.
In order to directly tin the wire, the lacquer insulation is spontaneously removed as the insulated wire is dipped into a hot solder bath and the bared wire is simultaneously coated with solder tin.
This behaviour makes it possible to tin the connecting ends of lacquered wire without the mechanical stripping that would otherwise be necessary and which would also be barely possible technically with the fine and ultra-fine wires in the above named fields. The mechanism of this tinning process involves the direct attachment of tin to the copper wire at the usual tinning temperatures of about 370.degree. C. The economic possibilities brought about by this feature of dispensing with mechanical skinning have considerable importance particularly for the broadcasting industry in which each piece of its equipment must be finished with a larger number of soldered junctions.
For over 20 years, this field of insulated wire has been dominated by lacquered wires having polyurethane coats. Polyurethane provides a particularly good coat for directly tinnable insulated wires since the urethane groups are reversibly cleaved at the high temperatures of the soldering bath. As a result of this cleavage, low molecular weight products are formed which function as a flux for the solder tin.
The basic polyurethane lacquer contains a resin component which has at least two free hydroxyl groups per molecule. Generally, this resin is a hydroxylated saturated polyester. The lacquer also has a blocked isocyanate having at least two functional isocyanate groups and a resin-like consistency.
Examples of suitable polyesters include acids like phthalic anhydride or isophthalic acid, or alcohols like ethylene glycol, glycerin or trimethylol propane, in which the hydroxyl group content of these polyesters is about 10 - 15% by weight of the polyester. As a polyisocyanate, a trifunctional isocyanate produced from trimethylolpropane and toluylene-2,4-diisocyanate is almost exclusively used. This trifunctional isocyanate is reacted with phenol in order to block the activity of the free isocyanate groups. The trifunctional isocyanate has the following structural formula: ##STR1##
This trifunctional isocyanate is commercially available under the trademark "Desmodur" AP stable from Farbenfabriken Bayer A.G. At present, this isocyanate is practically the sole isocyanate which can bestow the features required for polyurethane varnishes used to produce directly tinnable wire. The isocyanate bestows the important feature of a complete tinning capacity under conditions normally encountered in practice of a soldering bath (375.degree. C) in a short period of time (preferably not more than two seconds). An excellent elasticity is also bestowed. This trifunctional isocyanate has been important in the field of polyurethane varnishes because it has made advances possible in production of single-component varnishes. Such varnishes are necessary for electrical insulating varnish.
The varnish must be composed of ingredients which remain stable at the usual temperatures of the lacquering apparatus and react only at the elevated temperatures present during the heating process.
Furthermore the single-component lacquer must be of constant viscosity as far as possible at normal temperature if one is to obtain a uniformly thick film coating.
The requirement of providing a varnish which reacts only at the elevated temperatures of the heating process is satisfied by blocking the activity of the free isocyanate groups at the usual temperature of the lacquering apparatus. The free isocyanate groups are masked or shielded in order to prevent premature reaction with the alcoholic hydroxyl groups by reacting the isocyanate groups with phenol. Consequently, the mixture of hydroxyl-containing polyester and trifunctional isocyanates can be stored indefinitely at both standard and slightly elevated temperatures. This masking is also known as blocking or hindering. The masking becomes inoperative when sufficient heating (at temperatures above 130.degree. C) causes the isocyanate derivative to dissociate to the original phenol and isocyanate. The freed isocyanates then react with the hydroxyl groups of the polyester to produce polyurethane resin.
Substances other than phenol have been used as masking agents, but have acquired little significance for solderable wire varnishes. Alcohols have not been accepted as masking agents because of the excessive cleavage temperature which is required. As a practical matter, this high cleavage temperature would adversely affect the discharge speed of the coated wire from the lacquering apparatus.
At present, the lacquering solutions generally contain a mixture of phenols and isomers of cresols and xylenols as the principal solvents for the polyester-isocyanate mixture. These solvents are highly toxic and evaporate during heating together with the cleaved phenolic masking agent. Expensive antipollution equipment is required in order to avoid serious environmental pollution.